This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Mitochondrial myopathy, Encephalomyopathy, Lactic acidosis, with Stroke-like episodes (MELAS) syndrome is a disorder of mitochondrial dysfunction that has a broad spectrum of symptoms and variable age of onset. Common manifestations include exercise intolerance, stroke-like episodes, seizures, ragged red fibers, lactic acidosis, and dementia, with 75% of cases presenting before 20 years of age. The prevalence of stroke-like episodes approaches 99%. MELAS syndrome is one of the most frequent maternally inherited mitochondrial disorders with a minimal prevalence of 16.3/100,000. The pathogenesis of the stroke-like episodes is not clear. It is believed that epithelial dysfunction may lead to nitric oxide deficiency and ischemic events in the microvasculature. Both arginine and citrulline act as nitric oxide precursors. It has been reported that plasma arginine and citrulline levels are lower in subjects with MELAS syndrome and that administration of arginine leads to clinical improvement in the acute episodes and in the interictal state. However, there are no clinical studies evaluating the arginine flux and nitric oxide production or the effect of the administration of arginine or citrulline on nitric oxide production in subjects with MELAS syndrome. Thus, we designed a case control prospective study to determine arginine flux and nitric oxide production via a stable isotope infusion technique in subjects with MELAS syndrome and control subjects, both cohorts will have a baseline study without any supplementation of arginine or citrulline, and subjects with MELAS syndrome will be studied with arginine and with citrulline supplementation. The aims are to see whether nitric oxide production is lower in subjects with MELAS syndrome, whether arginine and citrulline supplementation will increase the nitric oxide production, and whether citrulline supplementation will increase nitric oxide production more than arginine supplementation secondary to arginine compartmentalization. If we were to demonstrate that subjects with MELAS syndrome have lower nitric oxide production than control subjects and that arginine and/or citrulline supplementation increases nitric oxide production, with either arginine or citrulline having a superior effect, the outcome of this clinical research will establish evidence to the usefulness of such supplementation in the treatment and prevention of stroke-like episodes in subjects with MELAS syndrome. Hypothesis (A): Subjects with MELAS syndrome have epithelial dysfunction altering the nitric oxide metabolism, leading to decreased nitric oxide production. Hypothesis (B): Administration of L-arginine and L-citrulline, as nitric oxide precursors, will increase production of nitric oxide in subjects with MELAS syndrome. Hypothesis (C): Arginine is compartmentalized in a sub-cellular compartment where arginine metabolism takes place, thus there may not be free exchange of circulating arginine. Rather it depends on the local citrulline pool to replenish the arginine pool for nitric oxide synthesis. Therefore L-citrulline supplementation will increase nitric oxide production to higher degree than L-arginine supplementation. Specific aim (A): To determine whether subjects with MELAS syndrome have lower nitric oxide production than control subjects. Specific aim (B): To test whether administration of L-arginine or L-citrulline to subjects with MELAS Syndrome will increase nitric oxide production. Specific aim (C): To determine whether L-citrulline supplementation will increase nitric oxide production to a higher degree than L-arginine supplementation.